Directly generating plasma using a gas under atmospheric pressure or a higher pressure eliminates the need for a high-vacuum device or similar equipment, and makes it possible to utilize plasma readily and inexpensively. It also becomes possible to utilize plasma in a wider range of industrial fields.
Although small-scale plasma can be generated using discharging or another method in which a spark plug or similar device is used, the plasma deactivates quickly under high pressures, making it difficult to generate a plasma on a large scale.
With the foregoing circumstances in view, attempts have been made to generate plasma under high pressures that equal or exceed atmospheric pressure. For example, a technique for generating and maintaining a plasma under a high pressure using a plasma torch is described in patent reference 1. According to this technique, a source gas is heated to an extremely high temperature using high-frequency waves, and a thermal plasma is generated.
A technique for generating a plasma in a supercritical fluid environment using an electrical discharge or a laser is described in patent reference 2.
Described in patent reference 3 is an apparatus that pressurizes sulfur hexafluoride (SF6) and helium to about 100 atmospheres and generates a plasma using a dielectric-barrier discharge, with the pressurized gas used as a source gas.    Patent reference 1: Japanese Laid-open Patent Publication No. 2004-216231    Patent reference 2: Japanese Laid-open Patent Publication No. 2003-178900    Patent reference 3: Japanese Laid-open Patent Publication No. 2006-004684